Rav said: the Evil Inclination resembles a fly and dwells between the two entrances of the heart, as it says, "Dead flies make the ointment of the perfumers fetid and putrid." And Shmuel said: It is like a kind of wheat, as it says, "Sin coucheth at the door..."

Rav Yehuda said in Rav's name: A man once conceived a passion for a certain woman, and his heart was consumed by his burning desire [his life being endangered thereby]. When the doctors were consulted, they said, 'His only cure is that she shall submit.' Thereupon the Sages said: 'Let him die rather than that she should yield.' Then [said the doctors]; 'let her stand nude before him;' [they answered] 'sooner let him die'. 'Then', said the doctors, 'let her converse with him from behind a fence'. 'Let him die,' the Sages replied 'rather than she should converse with him from behind a fence.' Now R. Yaakov bar Idi and R. Shmuel bar Nahmani dispute therein. One said that she was a married woman; the other that she was unmarried. Now, this is intelligible on the view, that she was a married woman, but on the latter, that she was unmarried, why such severity? — R. Papa said: Because of the disgrace to her family. R. Aha the son of R. Ika said: That the daughters of Israel may not be immorally dissolute. Then why not marry her? — Marriage would not assuage his passion, even as R. Yitzchak said: Since the destruction of the Temple, sexual pleasure has been taken [from those who practice it lawfully] and given to sinners, as it is written, "Stolen waters are sweet, and bread eaten in secret is pleasant."

They said: Since this is a time of Grace, let us pray for mercy for the Evil Inclination. They prayed for mercy, and he was handed over to them. He said to them: Realize that if you kill him, the world goes down. They imprisoned him for three days, then looked in the whole land of Israel for a fresh egg and could not find it. Thereupon they said: What shall we do now? Shall we kill him? The world would then go down. Shall we beg for half-mercy? They do not grant ‘halves’ in heaven. They put out his eyes and let him go. It helped inasmuch as he no more entices men to commit incest.

(7) Rabbi Nahman said in Rabbi Samuel's name: 'Behold, it was good' refers to the Good Desire; 'And behold, it was very good' refers to the Evil Desire. (It only says 'very good' after man was created with both the good and bad inclinations, in all other cases it only says 'and God saw that it was good') Can then the Evil Desire be very good? That would be extraordinary! But without the Evil Desire, however, no man would build a house, take a wife and beget children; and thus said Solomon: 'Again, I considered all labour and all excelling in work, that it is a man's rivalry with his neighbour.' (Ecclesiastes 4:4).

Surely it was taught, R. Simeon b. Eleazar said in the name of Halfa b. Agra in R. Johanan b. Nuri's name: He who rends his garments in his anger, he who breaks his vessels in his anger, and he who scatters his money in his anger, regard him as an idolater, because such are the wiles of the Evil Inclination: Today he says to him, 'Do this';tomorrow he tells him, 'Do that,' until he bids him, 'Go and serve idols,' and he goes and serves [them]. R. Abin observed: What verse [suggests this]? "There shall be no strange god in thee; neither shalt thou worship any strange god"; Who is the strange god that resides in man himself? Say, that is the Evil Inclination!

Antoninus also enquired of Rabbi, 'From what time does the Evil Inclination hold sway over man, from the formation [of the embryo], or from [its] issuing forth [birth]? 'From the formation,' he replied. 'If so,' he objected, 'it would rebel in its mother's womb and go forth [prematurely]. But it is from when it issues [birth].' Rabbi said: This thing Antoninus taught me, and Scripture supports him, for it is said, "At the door [i.e.,where the baby emerges] sin lies in wait."

[The explanation is] as R. Judah expounded: In the time to come the Holy One, blessed be He, will bring the Evil Inclination and slay it in the presence of the righteous and the wicked. To the righteous it will have the appearance of a towering hill, and to the wicked it will have the appearance of a hair thread. Both the former and the latter will weep; the righteous will weep saying, ‘How were we able to overcome such a towering hill!’ The wicked also will weep saying, ‘How is it that we were unable to conquer this hair thread!’

Even so did the Holy One, blessed be He, speak unto Israel: ‘My children! I created the Evil Inclination, but I [also] created the Torah, as its antidote; if you occupy yourselves with the Torah, you will not be delivered into his hand, for it is said: "If you do good, you shall be exalted." But if you do not occupy yourselves with the Torah, ye shall be delivered into his hand, for it is written, "sin crouches at the door."

Our Rabbis taught: V'-samtem [reads] samtam [a perfect remedy]. This may be compared to a man who struck his son a strong blow, and then put a plaster on his wound, saying to him, ‘My son! As long as this plaster is on your wound you can eat and drink at will, and bathe in hot or cold water, without fear. But if you remove it, it will break out into sores.’ Even so did the Holy One, blessed be He, speak unto Israel: ‘My children! I created the Evil Desire, but I [also] created the Torah, as its antidote; if you occupy yourselves with the Torah, you will not be delivered into his hand, for it is said: If thou doest well, shalt thou not be exalted? But if ye do not occupy yourselves with the Torah, ye shall be delivered into his hand, for it is written, sin coucheth at the door. Moreover, he is altogether preoccupied with thee [to make thee sin], for it is said, and unto thee shall be his desire. Yet if thou wilt, thou canst rule over him, for it is said, and thou shalt rule over him.

Kiddushin 30b

Rabbi Yitzhak said a person's yetzer renews upon him every day, as it is written "only evil all day." And Rabbi Shimon the son of Levi said a person's yetzer overcomes him every day and seeks to kill him, and if it were not for the Holy One's help a person could not withstand it.

Kiddushin 30b

The yetzer ha'ra is difficult, for even its Creator called it "bad", as it is written "for the yetzer of a person's heart is evil from his youth."

(21) And the LORD smelled the sweet savour; and the LORD said in His heart: ‘I will not again curse the ground any more for man’s sake; for the imagination (yetzer) of man’s heart is evil (rah) from his youth; neither will I again smite any more every thing living, as I have done.

(א) "מנעריו" - מנעריו כתיב משננער לצאת ממעי אמו נתן בו יצר הרע

Rashi on Genesis 8:21

"From his youth" It says from his youth, (m'n'urayv, root word נער) from when he shook (n'na'er, root word נער) to come out of his mother's innards, the yetser ha'ra was given in him.

Aruch HaShulchan 1:1:1-3

(1) Angels serve their Creator, and do not have yetzer ha'ra. Animals, on the other hand, have yetzer ha'ra but lack intelligence. The result is that angels cannot receive reward for their service, as they have no yetzer ha'ra to overcome, and animals cannot be punished for their actions, as they do not have the intelligence.(2) Therefore God found it necessary to create the human on the sixth day, and God created the human from both sides: God gave him a spiritual soul that enables him to recognize the Creator as does an angel, as the verse states: “God's candle is the human soul…” (Proverbs 27:20). And God created him in a body which is thick material, like a beast, to eat and drink and sleep. As a result, a great battles wage within a person all his days. His animal spirit incites him to desire this world like an animal, and the pure soul battles against him and shows him that he was not born for this, but to serve the Creator like an angel. And even physical things that one must do, like eating and drinking and sleeping -- should be done so as to to serve God.

Sefat Emet - Shabbat Chanukah 5640

For the core of everything is the power of the Blessed Creator, which is truth. But there is language and shell on the outside, and that is just depiction and imagination, and it is called yetzer ha'ra.